1. Field of the Invention
The present invention relates generally to the field of logarithmic amplifiers. More particularly, the present invention relates to a high performance, low power logarithmic amplifier cell for use in a multi-stage logarithmic amplifier providing accurate logarithmic intercept and slope response.
2. Discussion of the Related Art
Until recently, logarithmic amplifiers were practically unavailable which are useful for both low frequency and high frequency applications, and which are inherently free of sensitivity to temperature variations and production tolerances of the devices comprising the active elements so as to provide high accuracy log-law performance over a wide temperature range and in the presence of large production variations.
U.S. Pat. Nos. 4,929,909 and 4,990,803 (hereafter "the '909 patent" and "the '803 patent", respectively), assigned to the same assignee as the present invention and incorporated herein by reference, describe a logarithmic amplifier which does not suffer from the above-noted limitations.
FIG. 1 is a block diagram of the logarithmic amplifier disclosed in the '803 patent. The amplifier includes a plurality of amplifier/limiter/detector stages (hereinafter referred to as "gain" stages or "logarithmic amplifier gain stages") 100-1 through 100-n, a gain bias generator 102, a scale bias generator 104, and an offset generator 106, which are common to all gain stages. All of the gain stages working together produce a logarithmic output signal corresponding to the input signal. FIG. 2 is a block diagram for each of the gain stages 100-1 through 100-n. Each gain stage 100-i includes an emitter follower 112-i, having a differential input and balanced output, a full-wave detector 114-i, and a limiting amplifier 116-i. The emitter follower 112-i introduces a slight gain loss of approximately 0.07 dB. The limiting amplifier 116-i can be designed to provide a gain of 10.07 dB, so that the overall stage gain is 10 dB.
Although the logarithmic amplifier of the '909 and '803 patents performs well, that demodulating logarithmic amplifier requires relatively complicated gain stages having a relatively large number of active components, both positive and negative power supplies and requires approximately 300 milliwatts of power for a ten stage amplifier.